Conventionally, a lithography method has been used in fabricating fine structures in a various kinds of electronic devices such as semiconductor devices and liquid crystal devices. However, the need for micro-fabrication of resist patterns in lithography processes has arisen as device structures have been miniaturized.
At present, in the most advanced areas nowadays, for example, the lithography method can form a fine resist pattern which has a line width of about 90 nm; however, a finer pattern formation will be desired in the near future.
To achieve micro-fabrication of patterns of line widths finer than 90 nm, the success of the development of a photolithography machine and a resist suitable for the machine is the most important key. In the photolithography machines, development has generally been focused on shortening the wavelengths of light sources, such as F2 excimer lasers, EUV (extreme ultraviolet radiation), electron beams, X-rays, and soft X-rays, as well as on increasing the numerical aperture (hereinafter, simply referred to as NA) of lenses.
However, shortening of the wavelengths of light sources requires a new, expensive photolithography machine and further, increasing the NA raises the problem that the depth of focus is reduced even if resolving ability is improved because of the trade-off lies between resolving ability and depth of focus.
Recently, as a lithography technique enabling the solution of such problems, a method referred to as a liquid immersion lithography has been reported (for example, see Non-patent documents 1 to 3). In this method, a predetermined thickness of liquid refractive index medium (refractive index liquid or immersion liquid) such as deionized water or fluorinated inert liquid is placed on at least a resist film between the lens and the resist film on a substrate during exposure. In this method, instead of a conventional inert gas such as air or nitrogen used in a conventional method, a liquid with a larger refractive index (n), for example, deionized water is placed in the exposing light beam path space so as to achieve higher resolving ability and no reduction in depth of focus with the use of a light source with the same wavelength as the conventional one even if shorter wavelengths or any high NA lenses are used.
This liquid immersion lithography draws special attention, because it enables the formation of resist patterns with a higher resolving ability and superior depth of focus at a lower cost even if the lenses attached to the existing machine are used.
Non-patent document 1: Journal of Vacuum Science & Technology B (J. Vac. Sci. Technol. B) ((published in) U.S.A.), 1999, Vol. 17, No. 6, 3306-3309);
Non-Patent Document 2: Journal of Vacuum Science & Technology B (J. Vac. Sci. Technol. B) ((published in) U.S.A.), 2001, Vol. 19, No. 6, 2353-2356); and
Non-patent Document 3: Proceedings of SPIE ((published in) U.S.A.), 2002, Vol. 4961, 459-465.
However, in the liquid immersion lithography process, the resist film is directly in contact with the refractive index liquid (immersion liquid) during exposure, and hence the resist film is vulnerable to invasion by the liquid. Thus, conventional resist compositions need to be tested to determine whether they can be used in the liquid immersion lithography.
Currently used resist compositions have been established through extensive searches for resins providing with the most essential property which has transparency to the exposure light. The present inventors have conducted experiments in an effort to obtain resist compositions having suitable properties for use in liquid immersion lithography and to determine if conventional resist compositions can be used in liquid immersion lithography with or without slight modifications. As a result, it has been found that promising resist compositions from a practical point of view are available. On the other hand, it has been confirmed that there are a number of resist compositions which cannot achieve sufficient pattern resolution in liquid immersion lithography due to a change in their properties by the liquid, but which exhibit finer and higher resolving ability in ordinary lithography by exposure through a layer of air. Any of these resist compositions has been developed by expending significant development resources, and has various favorable resist properties, including transparency to the exposure light, developability and storage stability. The only disadvantage of these resist compositions is the lack of resistance to immersion liquid. Some examples of compositions which are not suitable for the liquid immersion lithography but achieve higher resolving ability in the lithography through a layer of air are shown in the Comparative Examples of the present invention described below.
Even for the aforementioned resist film intended for use in liquid immersion lithography, the quality and yield of non-defective products have been proven to be lower when the compositions are used in liquid immersion lithography than when used in common lithography in which the resist films are exposed via an air layer.
To evaluate the aforementioned requirements of conventional resist films for the liquid immersion lithography, the following analysis is conducted.
Specifically, for evaluating the formation of a resist pattern by liquid immersion lithography, it is considered to be necessary and sufficient to confirm three points: (i) the performance of the optical system in the liquid immersion lithography method,
(ii) the effect of the resist film on the immersion liquid, and
(iii) the change of properties of the resist film due to the immersion liquid.
(i) As clearly understood, for example, in the case of where is assumed where a photosensitive plate for photography, whose surface is water-proof, is immersed into water and a patterning light beam is irradiated onto the surface, there is in principle no doubt with respect to the point (i), that is the performance of an optical system, that no problem occurs if no light transmission loss occurs on the water surface and at the interface between the water and the surface of the photosensitive plate. In this case, light transmission loss may be easily solved by suitably correcting the incidence angle of the exposing light beam. Therefore, it is considered that any objects of exposure, for example, a resist film, a photographic sensitive plate, and an image screen, which are inert to the immersion liquid, namely, which are not affected by the immersion liquid, and which do not affect the immersion liquid, cause no change of the performance of the optical system. Thus, it is not necessary to further confirm this point or conduct any experiment on it.
(ii) The effect of the resist film on the immersion liquid specifically indicates that the component of the resist film is dissolved in the immersion liquid to change the refractive index of the liquid. Theoretically, when the refractive index of the immersion liquid changes, the optical resolution of the pattern exposure is sure to change and experiments are not necessary. In this point, it is enough to simply check whether the component of the resist film immersed in a liquid is dissolved in the liquid to change the formulation or refractive index of the immersion liquid, and it is not necessary to check resolving ability by actual irradiation of a pattern light and development.
Conversely, when the resist film immersed in the liquid is irradiated with a pattern light and developed to check resolving ability, it is possible to know as to whether resolving ability is excellent or poor, but it is difficult to judge whether resolving ability is affected by the change of the properties of the immersion liquid, the resist material, or both.
(iii) With respect to the phenomenon in which resolving ability is lowered by the change of properties of the resist film due to the immersion liquid, an evaluation test such as that in which “the resist film after exposure is showered with the immersion liquid and then developed, and the resultant resist pattern is examined in respect of resolving ability” is sufficiently evaluated. In this evaluation method, the resist film is directly showered with the liquid, and hence the conditions for immersion are very stringent. In this point, by the test in which the resist film completely immersed in the liquid is exposed, it is difficult to judge whether resolving ability is changed by the change of properties of the immersion liquid, the change of properties of the resist composition due to the immersion liquid, or by both.
The above phenomena (ii) and (iii) are two sides of the same coin, and may be figured out by identifying an alteration level of the resist film by the liquid.
Based on the results of the analysis, the suitability for liquid immersion lithography of the resist film currently proposed was evaluated by an evaluation test problem “the resist film after exposure is showered with the immersion liquid and then developed, and the resultant resist pattern is examined in respect of resolving ability.” The suitability can also be evaluated by simulating the practical production process using a “two-beam interferometry exposure method” that includes using an interfered light caused by a prism as a pattern light for exposure and subjecting a sample immersed in a liquid to exposure.
As mentioned above, the production of new resist films suitable for use in liquid immersion lithography require significant development resources. However, it has been demonstrated that some of the conventional resist compositions that have been suggested has properties suitable for the liquid immersion lithography with or without slight modifications, although their quality may deteriorate to some degree. It has also been demonstrated that many resist films are susceptible to alteration and fail to provide sufficient pattern resolution due to immersion liquid in liquid immersion lithography, but such resist films can still be used in lithography by exposing through common air layer to form fine patterns with higher resolving ability.
The present invention has been made in view of such problems associated with conventional techniques. An objective of the present invention is to provide a technique enabling the application of the resist film obtained from conventional resist compositions, which have required many development resources to the liquid immersion lithography. More specifically, the objective of this invention is to enable the formation of a resist pattern with higher resolving ability using liquid immersion lithography by forming a specific preventive film on the surface of the conventional resist film so as to simultaneously prevent the deterioration of the resist film and that of the immersion liquid used during the liquid immersion lithography.